The Engineer: High ratios not necessary for sufficient daylighting

Rational ratios
David Heska
February 05, 2018
By David Heska
As we kick off the new year I’m excited to begin this column.  The window industry is changing and we are seeing unprecedented pressure to manufacture better windows with lower U-values. However, along with this pressure there are many opportunities. At the WinDoor show held in November at the International Centre in Mississauga there were many great presentations regarding the trends in our industry.
It’s no secret that there is pressure from government, engineers and architects to decrease the window-to-wall ratio of our buildings, but there were great counterpoints made regarding the benefits that increased interior daylight has on occupant comfort and overall well-being.

Eighty per cent of the annual available interior daylighting can be achieved with a windowto-wall ratio of 40 per cent. Interestingly, increasing the ratio beyond that has relatively little effect on the amount of daylighting achieved. Even at 100 per cent glass, the interior receives only 85 per cent of all the available light hitting the building. We hear a lot about how healthy and energy-efficient it is to allow lots of light in. What we hear less is that it is possible to achieve this without making the entire wall out of glass.

At the same time as we realize this, we see that design trends over the last 50 years don’t acknowledge this reality. Since 1974, ratios have climbed from about 30 per cent to an average today of close to 60. I don’t think we will ever go back to the 30 per cent window-to-wall ratio of the 1970s, but the trend to design buildings with a ratio greater than 60 per cent cannot continue.

At a 50 per cent window-to-wall ratio, if the walls are R20 and the windows are R2 the effective R-value is only 3.6. If you take that same wall and increase the wall insulation to R40 (with R2 windows), your effective R-value only increases to 3.8. Now if you were to keep the walls at R20 and instead change the windows to R4 (U-value 0.25), your effective R-value jumps to 6.7. It’s very clear that installing higher-performing windows in our buildings will result in greater energy improvements. This needs to become the new standard and our industry needs to push the envelope by developing windows with U-values lower than 0.25. How low can we go? Will 0.20 become the new standard? Can we reach 0.15 or even 0.10?  In the coming decade, triple-glazed windows will become increasingly popular, double thermal breaks will become standard and more composite materials will be used.

We will have the wait and see how the government of Ontario’s recent “GreenON” rebates will impact the residential market. Homeowners are now eligible for $500 per window (to a maximum of $5,000) if high-performance Energy Star windows are installed. I am seeing and hearing that manufacturers are showing an increased desire to achieve the Energy Star standard.  In order to qualify the windows must have a U-factor at or below 0.25, but making homeowners aware of this rebate will be a challenge.  Having only a small list of contractors qualified to installed these windows will likely result in the rebate not having the desired wide-reaching effect. What would happen if the rebate were given to the window manufacturers rather than the homeowners? Can the industry be trusted to use these rebates for research and development instead of boosting profits?  

As we begin 2018 I challenge each reader to do their part in helping our industry build better buildings.  That includes reasonable window-to-wall ratios (around 40 to 50 per cent). It includes windows with U-values less than 0.25 and it includes each of us doing our part to creatively develop products that are more energy efficient.


David Heska, P.Eng. is a director with WSP’s Building Sciences team in southwestern Ontario. He oversees the operation of the Hamilton, Kitchener and Windsor offices. David has been involved on window simulation projects as well as the design and replacement of windows in commercial and residential buildings. He can be reached at
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